In a typical electrochemical gas sensor, the gas to be measured diffuses from the atmosphere through a gas porous membrane to a working electrode where a chemical reaction occurs. The type, rate, and efficiency of the chemical reaction is controlled by the material used to make the working electrode, the diffusion rate of the gas to the working electrode, and the potential at which the working electrode is set in relation to a reference, another electrode. The working electrode potential is commonly set with the aid of a potentiostat circuit, but this is not necessarily a required operating mode. At the counter electrode, a chemical reaction complementary to the one occurring at the working electrode takes place. The current flow between the working electrode and the counter electrode is proportional to the concentration of the gas being measured. An ionically conductive liquid electrolyte contacts all the electrodes and allows charge balance to be maintained within the sensor. Such electrochemical gas sensors are generally disclosed and described in U.S. Pat. Nos. 4,132,616; 4,324,632; 4,474,648; and in European Patent Application No. 0 496 527 A1.
An exploded view of a presently-known electrochemical gas sensor used for detecting carbon monoxide is shown in FIG. 1. During assembly of such a sensor, a platinum counter electrode 2 is placed on the inside bottom of a sensor housing 4. The counter electrode 2 typically includes a gas porous membrane 3 such as Gortex.RTM. or Zitex.RTM.. Next, a gold-coated current collector 6 is placed in the sensor housing 4 in a manner allowing the gold-plated ring to contact the perimeter of the counter electrode 2 and the tab to extend through a lower hole (not shown) in the sensor housing 4 on top of the current collector 6. An O-ring 8 is then placed in the sensor housing 4 with the main spacer 10 being placed on top of the O-ring 8.
With these pieces in place, an electrically insulating but porous separator 12 is placed within the main spacer 10 and then a wick 14 is placed over the separator 12. Preferably, the wick is dumbbell-shaped and made from porous polyethylene or polypropylene which has been treated to make it hydrophilic. A second O-ring 16 is then placed over the main spacer 10 and a second gold-coated current collector 18 is placed on top of this O-ring 16 with the tab extending through a middle hole (not shown) in the sensor housing 4. Next, a platinum reference electrode 20 which has a center hole 22 is placed over the current collector 18 in a manner allowing them to make electrical contact. A third O-ring 24 is then placed over the reference electrode followed by a spacer 26 and one or more separators 28. Separators 28 are similar to separator 12.
A third gold-coated current collector 30 is then placed over this assembly with its tab extending through an upper hole (not shown) in the sensor housing 4. A platinum working electrode 32 is placed over the current collector 30. The working electrode 32 is similar in structure to the counter electrode 2 and also includes a gas porous membrane such as Gortex.RTM. or Zitex.RTM.. The working electrode 32 is inserted face down whereas the counter electrode is face up.
The sensor inlet assembly 34 which includes a baffle 36 to reduce convection is then pushed down over the stack and forms the top of sensor housing 4. The entire structure is maintained under some pressure in sensor housing 4 by fitting a retaining ring 38 into a groove 40 at the top of the sensor housing 4. Later, the tabs of the current collectors which extend through the lower, middle and upper holes are bent parallel to, and heat sealed to, the outer wall of the sensor housing 4. The area around where the tabs protrude through the holes in the sensor housing 4 is then coated with a hydrophobic sealant. After this sealant has dried, the sensor is filled with an ionically conductive aqueous sulfuric acid electrolyte through a fill hole 42 near the bottom of the sensor housing 4 which is then sealed with plug 44.
Toxic gas sensors utilizing this configuration have several disadvantages. Among them are high manufacturing costs which are due to the numerous parts used in the sensor as well as the labor involved with assembling the sensor and with applying the heat seal and hydrophobic sealants. Additionally, the high cost of precious metals such as the gold in the current collectors requires the use of fragile, laminated leads which are not very sturdy and which must be protected from mechanical abuse while still allowing for reliable external electrical connections. Even with the use of gaskets or O-rings, and hydrophobic sealants, sensors of this type still tend to leak electrolyte after long periods of use or after exposure to elevated temperatures. The leakage of the liquid electrolyte, typically aqueous sulfuric acid, not only reduces the performance characteristics of the sensor but can also corrode and destroy the instrument in which the sensor is located. Still another drawback to this type of sensor is its size which is over one inch in height.
Toxic gas sensors such as the one shown in FIG. 1 and described above do not use gas permeable membranes because the permeability of well known materials to gases and vapors is either too low or the materials are not sufficiently inert to withstand typical toxic gas sensing environments. As a result, toxic gas sensors utilize gas porous membranes such as Gore-Tex.RTM. or Zitex.RTM.. These gas porous membranes are usually made out of PTFE (polytetrafluoroethylene) which contain a large number of microscopically visible holes which are on the order of several microns in diameter. These holes typically cover about 60-70% of the geometric area of the gas porous membrane.
Although electrochemical toxic gas sensors made with gas porous membranes work acceptably in many applications, they are generally acknowledged to have several drawbacks. For example, the porosity of a gas porous membrane, for the most part, limits the choice of acceptable liquid electrolytes which can be used in the sensor primarily to aqueous acids. Even when using such aqueous acids, application of a pressure differential to the sensor can cause the electrolyte to weep through the porous membrane.
Additionally, water vapor rapidly transpires through a gas porous membrane as temperature and humidity change. This makes it necessary to leave space in the body of the sensor for a relatively large electrolyte reservoir. This increases the complexity of the sensor and causes the sensor to be larger than desirable. It also causes the pH of the electrolyte to change which changes the potential of the reference electrode in the sensor. The drift in the potential of the reference electrode results in zero drift, span drift and temperature compensation problems when using the sensor in an instrument. Also, aerosols, particles, and high molecular weight gases can easily pass through the microscopic holes in a porous membrane, causing poisoning of the sensing electrode. This poisoning phenomena results in a slow decline in sensor output with time until the sensor is chemically destroyed and no longer useable.
It would be desirable, therefore, to have an electrochemical toxic gas sensor which did not have these drawbacks.